What if instead of Mii costumes with Kazuya, we instead get four echo fighters including Jin Kazama, while they save the Lloyd mii costume for Challenger Pack 11?
Adding on to that, who do you want those echo fighters to be?
My list
03e: Ocarina of Time Adult Link (Same as in Melee but with Gale Boomerang and Ice Arrows maybe)
14e: Dry Bowser (Melee/Brawl Bowser with Giga Bowser effects)
31e: Shadow (Different specials like Chaos control and Chaos Spear to replace spring jump and spin dash respectively)
81e: Jin Kazama (The mix of the mishima style and Jun Kazama's fighting style from Tekken 3)
I see you've gone to Cool School with that line of thinking. I TOO am an alumni of Cool School, and I love this kind of thinking.
My 4 echoes:
Dry Bowser as an Echo of Bowser (actually surprised it isn't already a thing)
Hilda as an echo of Zelda (same thing, especially with Zelda's design in Smash. Surprised she isn't already there)
Metal Sonic as an echo of Sonic (YOU MIGHT KNOW EVERYTHING I'M GOING TO DO, BUT THAT'S NOT GOING TO HELP YOU BECAUSE I KNOW EVERYTHING YOU'RE GOING TO DO! STRRRRRANGE ISN'T IT? GRRRRRRRRRRRRRRRRRR)
And yeah, if they could make Jin Kazama work, he'd be cool too, I had much too much fun with Devil Within to not want him. If not, then Gooigi as an echo of Luigi works too.